Self-assembly of surfactants

Although the notion of monomolecular surface layers is of fundamental importance to all phases of surface science, surfactant monolayers at the aqueous surface are so unique as virtually to constitute a special state of matter. For the many types of amphipathic molecules that meet the simple requirements for monolayer formation it is possible, using quite simple but elegant techniques over a century old, to obtain quantitative information on intermolecular forces and, furthermore, to manipulate them at will. The special driving force for self-assembly of surfactant molecules as monolayers, micelles, vesicles, or cell membranes (Fendler, 1982) when brought into contact with water is the hydrophobic effect. 

Introduction to the variety of types of surfactants, effect of surfactants on aqueous solution properties. Law of mass action applied to the self-assembly of surfactant molecules in water. Spontaneous self-assembly of surfactants in aqueous media. Formation of micelles, vesicles and lamellar structures. Critical packing parameter. Detergency. Laboratory project on determining the charge of a micelle. 

Chapter 8 discusses self-assembly of surfactants to form micelles, models of micelliza-tion, use of micelles in catalysis and solubilization, and oil-in-water and water-in-oil microemulsions. 

We have already seen from Example 10.1 that van der Waals forces play a major role in the heat of vaporization of liquids, and it is not surprising, in view of our discussion in Section 10.2 about colloid stability, that they also play a significant part in (or at least influence) a number of macroscopic phenomena such as adhesion, cohesion, self-assembly of surfactants, conformation of biological macromolecules, and formation of biological cells. We see below in this chapter (Section 10.7) some additional examples of the relation between van der Waals forces and macroscopic properties of materials and investigate how, as a consequence, measurements of macroscopic properties could be used to determine the Hamaker constant, a material property that represents the strength of van der Waals attraction (or repulsion see Section 10.8b) between macroscopic bodies. In this section, we present one illustration of the macroscopic implications of van der Waals forces in thermodynamics, namely, the relation between the interaction forces discussed in the previous section and the van der Waals equation of state. In particular, our objective is to relate the molecular van der Waals parameter (e.g., 0n in Equation (33)) to the parameter a that appears in the van der Waals equation of state ... 

Micelles are dynamic structures where a frequent exchange of monomers between micelles and the bulk solution occurs. Thermodynamics of the self-assembly of surfactants or aggregates is determined by the free energy of transfer of a surfactant monomer from water to the micelle. 

Figure 11.5 Self-assembly of surfactants in aqueous phase.

Considerations of the packing parameter concept of Israelachvili et al. [1] suggest that double-chain surfactants, which form the basis of measurements described in this article, cannot readily form spherical micelles. With double-chain surfactants, a more likely aggregate structure is the formation of bilayer vesicles, which can be also thought of as a dispersed lamellar phase (La) as such the vesicular dispersed form is likely to be preferentially formed at low concentrations ( 1 mmol dm-3) of surfactant. Furthermore, it is necessary to consider the possibility, unlike in the case of micelles, that such vesicles, formed by self-assembly of surfactant monomers, will not be thermodynamically stable. The instability is then likely to be in the direction of growth to a thermodynamically-stable lamellar phase from the vesicles. This process will be driven, at least initially, by fusion of two vesicles. 

Choi, K. S., Lichtenegger, H. C., Stucky, G. D. and McFarland, E. W. (2002). Electrochemical synthesis of nano structured ZnO films utilizing self-assembly of surfactant molecules at solid-liquid interfaces. J. Am. Chem. Soc. 124(42), 12402-12403. 

Vauthey, S., Santoso, S., Gong, H., Watson, N., and Zhang, S. (2002), Molecular self-assembly of surfactant-like peptides to form nanotubes and nanovesicles, Proc. Nat. Acad. Sci. U. S. A., 99, 5355-5360. 

Extension of the molecular sieves to the mesoporosity range is possible nsing lyotropic liquid crystal mesophases (Figine 25.20) as removable templates. These mesophases result from the self-assembly of surfactants or amphiphilic molecules and can be thermally or chemically eliminated after the formation of the inorganic network. This approach enables the preparation of materials exhibiting an ordered... 

The present section gives a brief overview of ideas and the physical notions behind self-assembly of surfactant-water systems. 

In dealing with self-assembly of surfactants and lipids, we must consider two aggregation states monolayers and bilayers. The latter class are ubiquitous in biological membranes, discussed in detail in Chapters 5 and 7. If the constituent monolayers in the bilayer are made up of identical molecules, the local geometry of both monolayers must be identical. In this case the mean... 

The typical synthesis of mesoporous material can be divided into two main stages (i) Formation of the organic-inorganic liquid-crystal phase (mesophases or mesostructure) results from the self-assembly of surfactant molecules and inorganic species which are polymerizable (or condensable) under synthesis conditions. Moreover, this mesostructure has a crystal lattice with the cell length in the nanometer range, (ii) Removal of surfactant from the mesostructure by calcination at high temperatures or other physical or chemical treatments results in the formation of mesopores (the space occupied by surfactant molecules) in the mesostructure. 

The control of morphology in mesoporous materials is thought to be governed by kinetic effects as the self-assembly of surfactant molecules and nucleation processes... 

Santoso, S., Hwang, W., Hartman, H. et al. (2002a). Self-assembly of surfactant-like... 

There has been much interest in studying surfactant aggregation in polar solvents other than water over the last few decades. In a large number of studies various surfactant systems have been mapped and evidence for self-assembly of surfactants in some nonaqueous polar solvents has been published. During the last few years more detailed information on the structure of the aggregates and on the characteristics of the aggregation processes have been provided. 

Surfactant-polymer interactions in an aqueous solution have been studied by many researchers [132], and the adsorption and surface-induced self-assembly of the surfactant at the solid-aqueous interface have been recently studied [133]. On the other hand, these subjects have been rarely studied for oil solutions. The surfactant-polymer interaction in oil and the surface-induced self-assembly of surfactants at the oil-solid interface are important for further research studies to enhance the polymerization at the interface of the liquid/solid in reversed micellar solutions. 

Of course, the self-assembly of surfactant molecules, whether it happens in a single solvent phase or in the presence of both oil and water, can lead to solidlike organized structures called liquid crystals, which are nevertheless nonstoichiometric [8]. 

Figure 19.10. Illustrations of the effects of hydrophobic interactions, i. e. the tendency to eliminate contacts between water and nonpolar molecules or surfaces (a) water and oil are immiscible, with a strong driving force to expel hydrocarbon molecules from water (b) self-assembly of surfactant molecules (c) other types of association of hydrocarbon chains (d) folding of proteins (e) strong adhesion between non-polar surfaces in water (f) non-wetting of water on hydrophobic surfaces (g) rapid coagulation of hydrophobic particles in water (h) attachment of hydrophobic particles to air bubbles (mechanism of froth flotation). (Redrawn from J. Israelachvili, Intermolecular and Surface Forces, with Applications to Colloidal and Biological Systems, Academic Press, London, 2nd Edn, 1991)...

For a review in this field, see F. Tiberg, J. Brinck, L. Grant, Adsorption and surface-induced self-assembly of surfactants at the solid-aqueous interface, Curr. Opin. Colloid Interf Sd. 2000, 4,411-419. 

The spontaneous self-assembly of surfactants is an active area of research in part because weak interactions in solutions of ionic surfactants depend on both ion type and charge and these ion specific effects alter, sometimes dramatically, the chemical and physical properties of surfactant solutions. However, consensus is absent on how to model these effects. In modem science terms this is an ancient problem because specific ion effects of surfactants solutions, proteins and biomembranes have been known for... 

---